For many years, joists have been a fundamental component of the construction industry, and they have a wide variety of forms depending on their particular load bearing application, such as supporting floor and roof systems.
In its most common form, a joist consists generally of two longitudinally extending chord members spaced from one another and interconnected by a web that extends between the chord members. In joists of this type which are intended for use over short spaces (e.g., up to sixty feet), the web of the joists is most frequently formed of a plurality of lengths of round steel bars which are bent into a predetermined serpentine or sinuous shape, and each such length is then welded to the top and bottom chord members at the several points where the apexes of the upper and lower curved portions of the web abut the top and bottom chord members. Again, in their most common form, each of the top and bottom chord members are formed of two spaced angle irons and the apexes of the curved portions of the sinuous web pieces are positioned between the spaced, flat wall portions of each pair of angle irons and welded thereto by one of several well-known welding techniques, usually gas shielded or flux cored welding.
Short span joists of this type are usually made by what is essentially a manual welding operation because the use of lengths of curved web pieces in the joists does not lend itself to more fully automated welding procedures. More specifically, the web piece is typically between four and six feet in length, and its sinuous curvature is such that the desired spacing between adjacent apexes is typically twenty-four inches, whereby it must be welded to the top chord member at a position along its length spaced every twenty-four inches, and the same welding must also be done at the same spacing along the bottom chord member. However, because of the inherent "spring back" in the sinuous curvature of the steel web pieces, the desired twenty-four inch spacing is not obtained on a consistently exact basis. This variation is acceptable in joists and is readily accommodated in a manual welding operation where the welding technician can position the welding equipment at whatever point the apex of the web piece abuts the chord members. However, in automatic welding operations, the welding equipment is generally fixed in position, and the significant variations caused by the aforesaid "spring back" can result in an automatic weld being made at a point where there is little or no abutment between web piece and the chord member.
The most common manual joist making machine operation for short span joists consists of using a jig for the chord member to properly locate the two pairs of angle irons in spaced relation to one another, and all of the sinuous lengths of the web material are individually laid into the jig manually so that the apexes of the web pieces are disposed between the top and bottom angle irons, and clamps are used to hold the web pieces to the angle irons at each point of abutment. In some of these joists the end portion may also be formed of one or two linear round metal bars which are also manually placed between the chord members at both ends thereof, adjacent the sinuous web portions and then clamped in place thereat. The assembled joist is then removed from the jig and taken from the assembly area to a welding area where welds are made at each point of abutment. In most long span joists, a similar manual forming operation is used, but the web may be made up of a plurality of linear lengths of angle iron which have both end portions crimped to reduce their width so that they will fit within the spacing (e.g., one inch) between the angle irons of the two chord members before they are manually welded to the angle irons.
In the past, it is also known that joists could be manually formed using two spaced pieces of wood as the chord members, and using a plurality of individual metal tubes as the webbing, these metal tubes having their end flattened and being arranged in an alternating "V" and inverted "V" pattern with the one flattened end of each adjacent tube overlapping that of an adjacent tube and being fixed to the wooden chord member by connecting rods inserted through the chord member and the overlapping end portion.
Forming joists by what is essentially a manual welding operation is time consuming and, more importantly perhaps, requires significant labor costs, particularly as to the costs of skilled welding technicians. Another disadvantage that can result from the use of sinuous lengths of web material in manual operations is that excess material, and therefore increased costs, is utilized even though it is not needed. In some joist designs, strength requirements dictate that the compression member in the web must be larger than the tension member, so when a sinuous web piece is used the steel bar stock from which it is made must be of a sufficient size to meet the strength requirements of the compression member portion, and since the web piece is made of a continuous piece of the steel bar stock having a constant diameter along its length, the tension member portion of the web piece is of a size greater than that required for its load bearing function.
The present invention provides a method and apparatus for automatically making a joist which overcomes the aforesaid drawbacks of known technology for making joists, and which provides a number of practical and commercial advantages.